JP7820599B2 - Nonaqueous electrolyte for lithium secondary battery and lithium secondary battery containing same - Google Patents

Description

This application claims the benefit of priority based on Korean Patent Application No. 10-2021-0131937, filed on October 5, 2021, and Korean Patent Application No. 10-2022-0115214, filed on September 13, 2022, and all contents disclosed in the documents of said Korean patent applications are incorporated herein by reference.

The present invention relates to a nonaqueous electrolyte for a lithium secondary battery and a lithium secondary battery containing the same.

Lithium secondary batteries are generally manufactured by forming an electrode assembly by interposing a separator between a positive electrode containing a positive electrode active material made of a lithium-containing transition metal oxide and a negative electrode containing a negative electrode active material capable of storing lithium ions. The electrode assembly is then inserted into a battery case, which is then filled with a non-aqueous electrolyte solution that acts as a medium for transferring lithium ions, and sealed.

Lithium secondary batteries can be miniaturized and have high energy density and operating voltage, making them applicable in a variety of fields, including mobile devices, electronic products, and electric vehicles. As the range of applications for lithium secondary batteries becomes more diverse, the physical properties required of them are also becoming increasingly strict. In particular, there is a demand for the development of lithium secondary batteries that can operate stably even under high voltage and high temperature conditions and have long life characteristics.

On the other hand, when a lithium secondary battery is operated under high voltage and/or high temperature conditions, PF ₆ ^- anions from lithium salts such as LiPF ₆ contained in the electrolyte may thermally decompose to generate Lewis acids such as PF ₅ , which react with moisture to generate HF. Such decomposition products such as PF ₅ and HF may not only destroy the coating formed on the surface of the electrode, but may also cause a decomposition reaction of the organic solvent. Furthermore, the electrolyte decomposition products may react with the decomposition products of the positive electrode active material to elute transition metal ions, and the eluted transition metal ions may be electrodeposited on the negative electrode, destroying the coating formed on the surface of the negative electrode.

If the electrolyte decomposition reaction continues on such a destroyed coating, battery performance will further deteriorate, so there is a need to develop secondary batteries that can maintain excellent performance even under high voltage and high temperature conditions.

Korean Patent Publication No. 10-2003-0061219

The present invention aims to solve the above problems by providing a non-aqueous electrolyte solution containing a compound with a structure in which a propargyl group is bonded to coumarin, which suppresses electrolyte decomposition reactions and enables the formation of a reinforced coating on the electrode, as well as a lithium secondary battery containing the same.

In one embodiment, the present invention provides a nonaqueous electrolyte solution for a lithium secondary battery, comprising a lithium salt, an organic solvent, and a compound represented by the following chemical formula 1:

In the above Chemical Formula 1,
R1 is an alkyl group having 1 to 10 carbon atoms,
R2 is an alkylene group having 1 to 10 carbon atoms,
L is a direct bond, —O—, —COO—, —RO—, or —R′COO—;
R and R' each independently represent an alkylene group having 1 to 10 carbon atoms,
m is an integer of 0 to 5.

In another embodiment, the present invention provides a lithium secondary battery comprising: a positive electrode containing a positive electrode active material; a negative electrode containing a negative electrode active material; a separator interposed between the positive electrode and the negative electrode; and a nonaqueous electrolyte solution for the lithium secondary battery.

The nonaqueous electrolyte solution of the present invention contains a compound with a coumarin structure that reacts with active oxygen at the positive electrode to effectively suppress positive electrode decomposition, and a structure in which a propargyl group is bonded, which is advantageous for forming a coating on the surface of the negative electrode. This not only enables the formation of a highly durable coating on the electrode, but also suppresses the electrolyte decomposition reaction and reduces the gas generation rate in batteries containing high-nickel positive electrode active materials. This also makes it possible to provide a lithium secondary battery with fundamentally improved electrochemical characteristics.

The present invention is described in more detail below.

Generally, anions contained in lithium salts such as _LiPF6 , which are widely used in electrolytes for lithium secondary batteries, form decomposition products such as hydrogen fluoride (HF) and _PF5 due to thermal decomposition or moisture. These decomposition products have acid properties and deteriorate the coating or electrode surface in the battery.

Transition metals in the positive electrode are easily eluted into the electrolyte due to decomposition products of the electrolyte and structural changes in the positive electrode caused by repeated charging and discharging. The eluted transition metals are then redeposited on the positive electrode, increasing the resistance of the positive electrode. Furthermore, if the eluted transition metals migrate to the negative electrode via the electrolyte, they are electrodeposited on the negative electrode, causing destruction of the solid electrolyte interphase (SEI) film and further electrolyte decomposition reactions, resulting in problems such as increased lithium ion consumption and resistance.

Furthermore, during initial activation of the battery, a protective coating is formed on the positive and negative electrodes due to electrolyte reactions. However, if the coating becomes unstable for the reasons mentioned above, further decomposition of the electrolyte occurs during charging/discharging or exposure to high temperatures, accelerating battery degradation and generating gas.

In particular, batteries containing positive electrode active materials with a high nickel content have improved initial capacity characteristics, but repeated charging and discharging can lead to increased lithium by-products and gas generation due to side reactions, which can intensify the decomposition reaction of the electrolyte.

To solve this problem, the inventors discovered that adding a compound represented by the following chemical formula 1, which has a structure in which a propargyl group is bonded to coumarin, to a non-aqueous electrolyte reduces the decomposition reaction of the electrolyte and suppresses the elution of transition metals and the generation of gas. In particular, they confirmed that the inclusion of the non-aqueous electrolyte according to the present invention not only improves the initial capacity of batteries containing high-nickel (High-Ni) positive electrode active materials, but also improves their durability during high-temperature storage and cycling.

The following describes each component of the present invention in more detail.

Nonaqueous Electrolyte The present invention provides a nonaqueous electrolyte for a lithium secondary battery, comprising a lithium salt, an organic solvent, and a compound represented by Chemical Formula 1.

Each ingredient is explained in detail below.

(1) Compound Represented by Chemical Formula 1 The nonaqueous electrolyte solution of the present invention contains a compound represented by the following Chemical Formula 1.

In the above Chemical Formula 1,
R1 is an alkyl group having 1 to 10 carbon atoms,
R2 is an alkylene group having 1 to 10 carbon atoms,
L is a direct bond, —O—, —COO—, —RO—, or —R′COO—;
R and R' each independently represent an alkylene group having 1 to 10 carbon atoms,
m is an integer of 0 to 5.

As lithium secondary batteries continue to be charged and discharged, active oxygen such as singlet oxygen and superoxide is generated at the positive electrode, which can degrade battery performance. The coumarin contained in the compound represented by Chemical Formula 1 has a higher reactivity to active oxygen than electrolyte solvents such as ethylene carbonate. Therefore, it reacts with active oxygen before the electrolyte solvent does, thereby suppressing decomposition of the electrolyte caused by active oxygen. Furthermore, the compound represented by Chemical Formula 1 contains a propargyl functional group, which is easily reduced, and thus can form a durable passive film on the surface of the negative electrode as it is reductively decomposed. In other words, because it can form a stable film at the interface between the positive and negative electrodes and the electrolyte, it can suppress side reactions and ensure stability even when using a high-nickel positive electrode active material. This effectively improves not only the initial performance of lithium secondary batteries, but also their high-temperature durability and long-term life.

In one embodiment of the present invention, the compound represented by Chemical Formula 1 may include at least one compound selected from the group consisting of compounds represented by the following Chemical Formula 1-1 and compounds represented by the following Chemical Formula 2-1.

In the chemical formula 1-1 and the chemical formula 2-1,
R1, R2, L, and m are as defined in Chemical Formula 1 above.

Preferably, the compound represented by Chemical Formula 1 may include a compound represented by Chemical Formula 1-1. When a propargyl group is substituted at the 3-position as in Chemical Formula 1-1, this is preferred because it has the effect of stabilizing radicals at the 3- and 4-positions and increasing reactivity.

In one embodiment of the present invention, L in Chemical Formula 1 may be -O- or -COO-.

In one embodiment of the present invention, the compound represented by Chemical Formula 1 may include at least one selected from the group consisting of compounds represented by Chemical Formula 1-2 below, compounds represented by Chemical Formula 1-3 below, compounds represented by Chemical Formula 2-2 below, and compounds represented by Chemical Formula 2-3 below. Preferably, the compound represented by Chemical Formula 1 may include at least one selected from the group consisting of compounds represented by Chemical Formula 1-2 below and compounds represented by Chemical Formula 1-3 below.

In the chemical formula 1-2, the chemical formula 1-3, the chemical formula 2-2, and the chemical formula 2-3,
R1, R2, and m are as defined in Chemical Formula 1. In one embodiment of the present invention, m in Chemical Formula 1 may be 0, and R2 may be an alkylene group having 1 to 5 carbon atoms, preferably an alkylene group having 1 to 3 carbon atoms, and more preferably a methylene group.

In one embodiment of the present invention, the compound represented by Chemical Formula 1 may include at least one selected from the group consisting of a compound represented by Chemical Formula 1A below, a compound represented by Chemical Formula 1B below, a compound represented by Chemical Formula 2A below, and a compound represented by Chemical Formula 2B below. Preferably, the compound represented by Chemical Formula 1 may include at least one selected from the group consisting of a compound represented by Chemical Formula 1A below and a compound represented by Chemical Formula 1B below.

In one embodiment of the present invention, the content of the compound represented by Chemical Formula 1 may be 0.1 wt % to 5 wt %, preferably 0.1 wt % to 1 wt %, and more preferably 0.2 wt % to 0.8 wt %, based on the total weight of the non-aqueous electrolyte.

When the content of the compound represented by Chemical Formula 1 is 0.1% by weight or more, it is preferable because it can achieve the effect of suppressing the decomposition of metal ions in the positive electrode that is the objective of the present invention. When the content is 5% by weight or less, it is preferable because it can prevent a decrease in battery performance due to an increase in resistance.

(2) Additives The nonaqueous electrolyte of the present invention may optionally further contain the following additives, as needed, to prevent the electrolyte from being decomposed in a high-voltage environment, which may cause electrode collapse, or to further improve low-temperature high-rate discharge characteristics, high-temperature stability, overcharge prevention, and the effects of suppressing battery expansion at high temperatures.

The additive may be one or more compounds selected from cyclic carbonate compounds, halogen-substituted carbonate compounds, sultone compounds, sulfate compounds, phosphate or phosphite compounds, borate compounds, nitrile compounds, amine compounds, silane compounds, benzene compounds, and lithium salt compounds.

The cyclic carbonate compound may be one or more compounds selected from vinylene carbonate (VC) and vinyl ethylene carbonate (VEC), and specifically may be vinylene carbonate.

The halogen-substituted carbonate compound may be fluoroethylene carbonate (FEC).

The sultone-based compound is a material capable of forming a stable SEI film on the surface of the negative electrode through a reduction reaction, and may be one or more compounds selected from 1,3-propane sultone (PS), 1,4-butane sultone, ethene sultone, 1,3-propene sultone (PRS), 1,4-butene sultone, and 1-methyl-1,3-propene sultone, and specifically 1,3-propane sultone (PS).

The sulfate-based compound is a material that can be electrically decomposed on the surface of the negative electrode to form a stable SEI film that does not crack even when stored at high temperatures, and may be one or more selected from ethylene sulfate (Esa), trimethylene sulfate (TMS), and methyl trimethylene sulfate (MTMS).

The phosphate or phosphite compound may be one or more selected from lithium difluoro(bisoxalato)phosphate, lithium difluorophosphate, tris(trimethylsilyl)phosphate, tris(trimethylsilyl)phosphite, tris(2,2,2-trifluoroethyl)phosphate, and tris(trifluoroethyl)phosphite.

The borate-based compound may be lithium tetraphenylborate.

The nitrile compound may be one or more selected from succinonitrile (SN), adiponitrile (ADN), acetonitrile, propionitrile, butyronitrile, valeronitrile, caprylonitrile, heptanenitrile, cyclopentanecarbonitrile, cyclohexanecarbonitrile, 2-fluorobenzonitrile, 4-fluorobenzonitrile, difluorobenzonitrile, trifluorobenzonitrile, phenylacetonitrile, 2-fluorophenylacetonitrile, 4-fluorophenylacetonitrile, ethylene glycol bis(2-cyanoethyl)ether (ASA3), 1,3,6-hexanetricarbonitrile (HTCN), 1,4-dicyano-2-butene (DCB), and 1,2,3-tris(2-cyanoethyl)propane (TCEP).

The amine compound may be one or more selected from triethanolamine and ethylenediamine, and the silane compound may be tetravinylsilane.

The benzene-based compound may be one or more selected from monofluorobenzene, difluorobenzene, trifluorobenzene, and tetrafluorobenzene.

The lithium salt-based compound is a compound different from the lithium salt contained in the non-aqueous electrolyte solution, and may be one or more compounds selected from lithium difluorophosphate (LiDFP; LiPO ₂ F ₂ ), lithium bisoxalatoborate (LiBOB; LiB(C ₂ O ₄ ) ₂ ), lithium tetrafluoroborate (LiBF ₄ ), and lithium difluoro(bisoxalato)phosphate (LiDFOP).

Preferably, the non-aqueous electrolyte according to one embodiment of the present invention may further contain one or more additives selected from vinylene carbonate (VC), vinyl ethylene carbonate (VEC), fluoroethylene carbonate (FEC), 1,3-propane sultone (PS), 1,3-propene sultone (PRS), ethylene sulfate (ESa), succinonitrile (SN), adiponitrile (ADN), ethylene glycol bis(2-cyanoethyl)ether (ASA3), 1,3,6-hexanetricarbonitrile (HTCN), 1,4-dicyano-2-butene (DCB), 1,2,3-tris(2-cyanoethyl)propane (TCEP), lithium difluorooxalatoborate (LiODFB), lithium tetrafluoroborate (LiBF ₄ ), lithium difluoro(bisoxalato)phosphate (LiDFOP), and lithium difluorophosphate (LiDFP).

More preferably, the nonaqueous electrolyte according to one embodiment of the present invention may further contain one or more additives selected from vinylene carbonate (VC), vinyl ethylene carbonate (VEC), 1,3-propane sultone (PS), ethylene sulfate (ESa), lithium difluorooxalatoborate (LiODFB), and lithium difluorophosphate (LiDFP).

Meanwhile, the content of the additive may be 0.1 wt % to 10 wt % based on the total weight of the non-aqueous electrolyte, and preferably 0.3 wt % to 5 wt %. When the content of the additive is within the above range, the side reaction suppression effect due to the formation of a coating on the positive and negative electrodes can be obtained.

(3) Organic Solvent The nonaqueous electrolyte solution of the present invention contains an organic solvent.

The organic solvent can be any of the various organic solvents commonly used in lithium electrolytes, without limitation. For example, the organic solvent may be a cyclic carbonate solvent, a linear carbonate solvent, a linear ester solvent, a cyclic ester solvent, a nitrile solvent, or a mixture thereof. Preferably, the organic solvent may include a mixture of a cyclic carbonate solvent and a linear carbonate solvent.

The cyclic carbonate solvent is a highly viscous organic solvent with a high dielectric constant, which allows for effective dissociation of the lithium salt in the electrolyte. It may be one or more solvents selected from the group consisting of ethylene carbonate (EC), propylene carbonate (PC), 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 2,3-pentylene carbonate, and vinylene carbonate, and preferably contains ethylene carbonate (EC) or propylene carbonate (PC).

The linear carbonate solvent is an organic solvent having low viscosity and low dielectric constant, and may be one or more selected from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, ethyl methyl carbonate (EMC), methyl propyl carbonate, and ethyl propyl carbonate, and preferably includes ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), or diethyl carbonate (DEC).

In order to produce an electrolyte solution with high ionic conductivity, it is preferable to use a mixture of a cyclic carbonate solvent and a linear carbonate solvent as the organic solvent.

The linear ester solvent may be one or more selected from methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, and butyl propionate, and preferably methyl propionate, ethyl propionate, or propyl propionate.

The cyclic ester solvent may be one or more selected from gamma-butyrolactone, gamma-valerolactone, gamma-caprolactone, sigma-valerolactone, and epsilon-caprolactone.

The nitrile solvent may be one or more selected from succinonitrile, acetonitrile, propionitrile, butyronitrile, valeronitrile, caprylonitrile, heptanenitrile, cyclopentanecarbonitrile, cyclohexanecarbonitrile, 2-fluorobenzonitrile, 4-fluorobenzonitrile, difluorobenzonitrile, trifluorobenzonitrile, phenylacetonitrile, 2-fluorophenylacetonitrile, and 4-fluorophenylacetonitrile, and preferably succinonitrile.

Unless otherwise specified, the remainder of the total weight of the nonaqueous electrolyte, excluding the organic solvent, other components, such as the compound represented by Chemical Formula 1, additives, and lithium salt, is the organic solvent.

(4) Lithium Salt The nonaqueous electrolyte solution of the present invention contains a lithium salt.

The lithium salt may be any of those commonly used in electrolytes for lithium secondary batteries, and is not limited thereto. Specifically, the lithium salt may contain Li ⁺ as a cation and F ⁻ , Cl ⁻ , Br ⁻ , I ⁻ , NO ₃ ⁻ , N(CN) ₂ ⁻ , BF ₄ ⁻ , ClO ₄ ⁻ , B ₁₀ Cl ₁₀ ⁻ , AlCl ₄ ⁻ , AlO ₄ ⁻ , PF ₆ ⁻ , CF ₃ SO ₃ ⁻ , CH ₃ CO ₂ ⁻ , CF ₃ CO ₂ ⁻ , AsF ₆ ⁻ , SbF ₆ ⁻ , CH ₃ SO ₃ ⁻ , (CF ₃ CF ₂ SO ₂ ) ₂ N ⁻ , (CF ₃ SO ₂ ) ₂ N ⁻ , (FSO ₂ ) ₂ N ⁻ , BF ₂ C ₂ O ₄ ⁻ , BC ₄ O ₈ ⁻ , BF ₂ C ₂ O ₄ CHF ⁻ , PF ₄ C ₂ O ₄ ⁻ , PF ₂ C ₄ O ₈ ⁻ , PO ₂ F ₂ ⁻ , (CF ₃ ) ₂ PF ₄ ^- , (CF ₃ ) ₃ PF ₃ ^- , (CF ₃ ) ₄ PF ₂ ^- , (CF ₃ ) ₅ PF ^- , (CF ₃ ) ₆ P ^- , C ₄ F ₉ SO ₃ ^- , CF ₃ CF ₂ SO ₃ ^- , CF ₃ CF ₂ (CF ₃ ) ₂ CO ^- , (CF ₃ SO ₂ ) ₂ CH ⁻ , CF ₃ (CF ₂ ) ₇ SO ₃ ⁻ , and SCN ⁻ may be included.

Specifically, the lithium salt may be LiPF ₆ , LiClO ₄ , LiBF ₄ , LiN(FSO ₂ ) ₂ (LiFSI), LiN(SO ₂ CF ₃ ) ₂ (LiTFSI), lithium bis(pentafluoroethanesulfonyl)imide (LiBETI), LiSO ₃ CF ₃ , LiPO ₂ F ₂ , lithium bis(oxalato)borate (LiBOB), lithium difluoro(oxalato)borate (Li The fluoroisotope may be one or more selected from lithium difluoro(oxalate)borate (LiFOB), lithium difluoro(bisoxalato)phosphate (LiDFOP), lithium tetrafluoro(oxalate)phosphate (LiTFOP), and lithium fluoromalonato(difluoro)borate (LiFMDFB), and preferably may be _LiPF6 .

In one embodiment of the present invention, the concentration of the lithium salt in the non-aqueous organic solution containing the lithium salt and an organic solvent may be 0.5 to 4.0 M, specifically 0.5 to 3.0 M, and more specifically 0.8 to 2.0 M. When the lithium salt concentration is within the above range, the effects of improving low-temperature output and cycle characteristics are sufficiently ensured, while excessive increases in viscosity and surface tension are prevented, and appropriate electrolyte impregnation is achieved.

Lithium Secondary Battery Next, the lithium secondary battery according to the present invention will be described.

The lithium secondary battery according to the present invention includes a positive electrode containing a positive electrode active material, a negative electrode containing a negative electrode active material, a separator interposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte solution, where the non-aqueous electrolyte solution is the non-aqueous electrolyte solution according to the present invention. As the non-aqueous electrolyte solution has been described above, a detailed description thereof will be omitted, and the following description will focus on the other components.

(1) Positive Electrode The positive electrode according to the present invention includes a positive electrode active material, and can be produced by coating a positive electrode current collector with a positive electrode slurry containing the positive electrode active material, a binder, a conductive material, a solvent, etc., followed by drying and rolling.

The positive electrode current collector is not particularly limited as long as it does not cause chemical changes in the battery and is conductive. For example, stainless steel; aluminum; nickel; titanium; baked carbon; or aluminum or stainless steel whose surface has been treated with carbon, nickel, titanium, silver, or the like may be used.

The positive electrode active material is a compound capable of reversible intercalation and deintercalation of lithium, and may specifically include a lithium composite metal oxide containing lithium and one or more metals such as cobalt, manganese, nickel, or aluminum.

More specifically, the positive electrode active material may be a lithium-cobalt-based oxide (e.g., LiCoO ₂ ), a lithium-manganese-based oxide (e.g., LiMnO ₂ , LiMn ₂ O ₄ ), a lithium-nickel-based oxide (e.g., LiNiO ₂ ), a lithium-nickel-manganese-based oxide (e.g., LiNi _1-Y Mn _Y O ₂ (where 0<Y<1), LiMn _2-z Ni _z O ₄ (where 0<Z<2)), a lithium-nickel-cobalt-based oxide (e.g., LiNi _1-Y1 Co _Y1 O ₂ (where 0<Y1<1)), a lithium-manganese-cobalt-based oxide (e.g., LiCo _1-Y2 Mn _Y2 O ₂ (where 0<Y2<1), LiMn _2-z1 Co _z1 O ₄ (where 0<Z1<2)), lithium-nickel-manganese-cobalt-based oxides (e.g., Li(Ni _p Co _q Mn _r1 )O ₂ (where 0<p<1, 0<q<1, 0<r1<1, p+q+r1=1) and Li(Ni _p1 Co _q1 Mn _r2 )O ₄ (where 0<p1<2, 0<q1<2, 0<r2<2, p1+q1+r2=2)), lithium-nickel-cobalt-transition metal (M) oxides (e.g., Li(Ni _p2 Co _q2 Mn _r3 M _s2 )O ₂ (wherein M is selected from the group consisting of Al, Fe, V, Cr, Ti, Ta, Mg, Ti, and Mo; and p2, q2, r3, and s2 are each atomic fractions of independent elements, satisfying the following conditions: 0<p2<1, 0<q2<1, 0<r3<1, 0<s2<1, and p2+q2+r3+s2=1). In particular, from the viewpoint of enhancing the capacity characteristics and stability of the battery, the positive electrode active material may include at least one selected from the group consisting of lithium-cobalt oxide, lithium-manganese oxide, lithium-nickel-manganese-cobalt oxide, and lithium-nickel-cobalt-transition metal (M) oxide, and preferably includes at least one selected from lithium-nickel-manganese-cobalt oxide having a nickel content of 55 atomic % or more and lithium-nickel-cobalt-transition metal (M) oxide having a nickel content of 55 atomic % or more.

Representative examples thereof include _at least one selected from the _{group consisting} of Li _{( Ni1 /3Mn1} / _{3Co1/ 3 ) O2} , Li( _{Ni0.6Mn0.2Co0.2} ) _O2 , Li( _{Ni0.5Mn0.3Co0.2} )O2, Li( _{Ni0.7Mn0.15Co0.15} ) _O2 , Li( _{Ni0.8Mn0.1Co0.1} ) _O2 , and _{Li ( Ni0.86Co0.05Mn0.07Al0.02 ) O2 ,} and _preferably Li _{( Ni0.86Co0.057Mn0.07Al0.02 ) O2 . 2} may be included.

Furthermore, the positive electrode active material according to one embodiment of the present invention may contain a lithium composite transition metal oxide represented by the following chemical formula 3:

[Chemical formula 3]
Li _1+x ( _Nia Co _b Mn _c M _d ) O ₂

In the above Chemical Formula 3,
M is one or more selected from W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo;
1+x, a, b, c, and d are each the atomic fractions of independent elements, specifically the atomic fractions of Li, Ni, Co, Mn, and M, respectively;
0≦x≦0.2, 0.50≦a<1, 0<b≦0.25, 0<c≦0.25, 0≦d≦0.1, and a+b+c+d=1.

Preferably, a, b, c, and d may be in the ranges 0.70≦a≦0.95, 0.025≦b≦0.20, 0.025≦c≦0.20, and 0≦d≦0.05, respectively.

Furthermore, a, b, c, and d may be in the ranges 0.80≦a≦0.95, 0.025≦b≦0.15, 0.025≦c≦0.15, and 0≦d≦0.05, respectively.

Furthermore, a, b, c, and d may be in the ranges 0.85≦a≦0.90, 0.05≦b≦0.10, 0.05≦c≦0.10, and 0≦d≦0.03, respectively.

The positive electrode active material may be contained in an amount of 80% to 99% by weight, specifically 90% to 99% by weight, based on the total weight of the solids in the positive electrode slurry. If the content of the positive electrode active material is less than 80% by weight, the energy density may be low, resulting in a decrease in capacity.

The binder is a component that aids in binding the active material and conductive material, and in binding them to the current collector, and is typically added in an amount of 1 to 30% by weight based on the total weight of the solids in the positive electrode slurry. Examples of such binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer, sulfonated ethylene-propylene-diene monomer, styrene-butadiene rubber, fluororubber, or various copolymers thereof.

The conductive material is a substance that provides conductivity without causing any chemical changes to the battery, and may be added in an amount of 0.5% to 20% by weight based on the total weight of solids in the positive electrode slurry.

Examples of the conductive material may be selected from the following conductive materials: carbon blacks such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; graphite powders such as natural graphite, artificial graphite, carbon nanotubes, and graphite; conductive fibers such as carbon fibers and metal fibers; conductive powders such as carbon fluoride powder, aluminum powder, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives.

The solvent for the positive electrode slurry may also include an organic solvent such as NMP (N-methyl-2-pyrrolidone), and may be used in an amount that provides a suitable viscosity when the positive electrode active material, binder, conductive material, etc. are included. For example, the positive electrode slurry containing the positive electrode active material, binder, and conductive material may be included so that the solids concentration is 40% to 90% by weight, preferably 50% to 80% by weight.

(2) Negative Electrode The negative electrode according to the present invention includes a negative electrode active material, and can be manufactured by coating a negative electrode slurry containing the negative electrode active material, a binder, a conductive material, a solvent, etc., on a negative electrode current collector, followed by drying and rolling.

The negative electrode current collector generally has a thickness of 3 μm to 500 μm. There are no particular restrictions on the negative electrode current collector, so long as it does not cause chemical changes in the battery and has high conductivity. Examples of materials that may be used include copper; stainless steel; aluminum; nickel; titanium; sintered carbon; copper or stainless steel surfaces treated with carbon, nickel, titanium, silver, or the like; and aluminum-cadmium alloys. As with the positive electrode current collector, the surface may be made finely uneven to strengthen the bonding strength of the negative electrode active material, and the negative electrode current collector may be used in various forms, such as a film, sheet, foil, mesh, porous material, foam, or nonwoven fabric.

The negative electrode active material may also include one or more selected from the group consisting of carbonaceous materials capable of reversibly intercalating/deintercalating lithium ions; metals or alloys of these metals with lithium; metal composite oxides; materials capable of doping and dedoping lithium; lithium metal; and transition metal oxides.

The carbonaceous material capable of reversibly intercalating/deintercalating lithium ions can be any carbonaceous negative electrode active material commonly used in lithium ion secondary batteries, without any particular limitations. Representative examples include crystalline carbon, amorphous carbon, or a combination of these. Examples of crystalline carbon include graphite, such as amorphous, plate-like, flake-like, spherical, or fibrous natural or artificial graphite. Examples of amorphous carbon include soft carbon (low-temperature calcined carbon), hard carbon, mesophase pitch carbide, and calcined coke.

The metals or alloys of these metals with lithium can be selected from the group consisting of Cu, Ni, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, and Sn, or alloys of these metals with lithium.

The metal composite oxide may be _one or more selected from the _group consisting of PbO, _PbO2 , _Pb2O3 , _Pb3O4 , _Sb2O3 , _Sb2O4 , _Sb2O5 , _GeO , _GeO2 , _{Bi2O3 , Bi2O4 , Bi2O5} , _{LixFe2O3 (0≦x≦1), LixWO2 (0≦x≦1), and SnxMe1-xMe'yOz ( Me : Mn , Fe , Pb ,} Ge; Me': Al, B, _P , _{Si ,} elements of Groups 1, 2 and 3 of the periodic table, halogens; 0<x≦1;1≦y≦3; 1≦z≦8).

Examples of the substance capable of doping and dedoping lithium include Si, SiO _x (0<x<2), Si-Y alloy (where Y is an element selected from the group consisting of alkali metals, alkaline earth metals, Group 13 elements, Group 14 elements, transition metals, rare earth elements, and combinations thereof, and is not Si), Sn, SnO ₂ , Sn-Y (where Y is an element selected from the group consisting of alkali metals, alkaline earth metals, Group 13 elements, Group 14 elements, transition metals, rare earth elements, and combinations thereof, and is not Sn), and at least one of these may be mixed with SiO ₂ for use. The element Y may be selected from the group consisting of Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db (dubnium), Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru, Os, Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, Ge, P, As, Sb, Bi, S, Se, Te, Po, and combinations thereof.

Examples of the transition metal oxide include lithium-containing titanium oxide (LTO), vanadium oxide, and lithium vanadium oxide.

The negative electrode active material may be contained in an amount of 80% to 99% by weight based on the total weight of solids in the negative electrode slurry.

The binder is a component that aids in bonding between the conductive material, active material, and current collector, and is typically added in an amount of 1 to 30% by weight based on the total weight of solids in the negative electrode slurry. Examples of such binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer, sulfonated ethylene-propylene-diene monomer, styrene-butadiene rubber, fluororubber, and various copolymers thereof.

The conductive material is a component for further improving the conductivity of the negative electrode active material and may be added in an amount of 0.5% to 20% by weight based on the total weight of the solids in the negative electrode slurry. There are no particular restrictions on the conductive material, so long as it does not cause chemical changes in the battery and is conductive. Examples of such conductive materials include carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, or thermal black; graphite powder such as natural graphite with a highly developed crystalline structure, artificial graphite, carbon nanotubes, or graphite; conductive fibers such as carbon fiber or metal fiber; conductive powders such as carbon fluoride powder, aluminum powder, or nickel powder; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; or conductive materials such as polyphenylene derivatives.

The solvent for the negative electrode slurry may include water or an organic solvent such as NMP or alcohol, and may be used in an amount that results in an appropriate viscosity when the negative electrode active material, binder, conductive material, etc. are contained. For example, the solvent may be contained so that the solids concentration in the slurry containing the negative electrode active material, binder, and conductive material is 30% to 80% by weight, preferably 40% to 70% by weight.

(3) Separator The lithium secondary battery according to the present invention includes a separator between the positive electrode and the negative electrode.

The separator separates the negative electrode and positive electrode and provides a path for lithium ions to move. Any separator commonly used in lithium secondary batteries can be used without any particular restrictions. In particular, separators that offer low resistance to the movement of ions in the electrolyte, excellent electrolyte impregnation ability, and safety are preferred.

Specifically, the separator may be a porous polymer film, such as a porous polymer film made of a polyolefin polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene/butene copolymer, an ethylene/hexene copolymer, or an ethylene/methacrylate copolymer; or a laminate structure of two or more layers of these. Ordinary porous nonwoven fabrics, such as nonwoven fabrics made of high-melting-point glass fibers or polyethylene terephthalate fibers, may also be used. Furthermore, to ensure heat resistance or mechanical strength, a coated separator containing a ceramic component or a polymeric substance may also be used, and may be used in a single-layer or multi-layer structure.

The lithium secondary battery according to the present invention as described above can be useful in portable devices such as mobile phones, laptop computers, and digital cameras, as well as in electric vehicles such as hybrid electric vehicles (HEVs).

Accordingly, according to another embodiment of the present invention, a battery module including the lithium secondary battery as a unit cell and a battery pack including the same are provided.

The battery module or battery pack can be used as a power source for one or more medium- to large-sized devices, including power tools; electric vehicles, including electric vehicles (EVs), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs); and power storage systems.

The external shape of the lithium secondary battery of the present invention is not particularly limited, but may be a cylindrical shape using a can, a rectangular shape, a pouch shape, a coin shape, or the like.

The lithium secondary battery of the present invention can be used not only as a battery cell used as a power source for small devices, but also as a unit battery in medium- to large-sized battery modules containing multiple battery cells.

The present invention will be explained in more detail below using specific examples.

<Example>
Example 1.
(Production of non-aqueous electrolyte)
Ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed in a volume ratio of 30:70, and _LiPF6 was dissolved therein to prepare a non-aqueous organic solution to a concentration of 1.0 M. 0.5 wt % of the compound represented by Formula 1A (Cas No. 119827-21-3), 0.5 wt % of vinylene carbonate (VC), 0.5 wt % of 1,3-propane sultone (PS), and the remainder of the non-aqueous organic solution were mixed together to prepare a 100 wt % non-aqueous electrolyte.

(Manufacturing of lithium secondary batteries)
A cathode active material ( _{LiNi0.85Co0.05Mn0.07Al0.03O2 ) ,} a conductive material (carbon _nanotubes ), and a binder (polyvinylidene fluoride) were mixed in a weight ratio of 98.0:0.7:1.3 to a solvent _, N-methyl-2-pyrrolidone (NMP), to prepare a cathode mixture slurry (solid content 76.5 wt%). The cathode mixture slurry was applied to one side of a 12 μm-thick cathode current collector (Al thin film), dried, and roll-pressed to prepare a cathode.

Anode active material (artificial graphite), conductive material (carbon black), and binder (styrene-butadiene rubber) were added to distilled water as a solvent in a weight ratio of 96.5:1.5:2.0 to prepare anode mixture slurry (solid content 50% by weight). The anode mixture slurry was applied to one side of an 8 μm-thick anode current collector (Cu thin film), dried, and roll-pressed to prepare anodes.

In a dry room, a polyethylene porous film separator was placed between the positive electrode and negative electrode prepared above, and the non-aqueous electrolyte prepared above was then injected to produce a secondary battery.

Example 2.
A lithium secondary battery was manufactured in the same manner as in Example 1, except that the compound represented by Chemical Formula 1B (Cas No. 71387-23-0) was used instead of the compound represented by Chemical Formula 1A when preparing the non-aqueous electrolyte.

Example 3.
A lithium secondary battery was produced in the same manner as in Example 1, except that VC and PS were not added during the production of the non-aqueous electrolyte solution.

Example 4.
A lithium secondary battery was manufactured in the same manner as in Example 1, except that LiCoO ₂ was used as the positive electrode active material instead of LiNi _0.85 Co _0.05 Mn _0.07 Al _0.03 O ₂ during the manufacture of the positive electrode.

Example 5
A nonaqueous electrolyte solution and a lithium secondary battery were manufactured in the same manner as in Example 1, except that the compound represented by Chemical Formula 2A was used instead of the compound represented by Chemical Formula 1A (Cas No. 119827-21-3) when manufacturing the nonaqueous electrolyte solution.

Example 6
A nonaqueous electrolyte solution and a lithium secondary battery were manufactured in the same manner as in Example 2, except that the compound represented by Chemical Formula 2B was used instead of the compound represented by Chemical Formula 1B (Cas No. 71387-23-0) when manufacturing the nonaqueous electrolyte solution.

Comparative Example 1.
A lithium secondary battery was fabricated in the same manner as in Example 1, except that the compound represented by Formula 1A was not added during the preparation of the non-aqueous electrolyte.

Comparative Example 2.
A lithium secondary battery was fabricated in the same manner as in Example 1, except that coumarin was used instead of the compound represented by Formula 1A when preparing the non-aqueous electrolyte.

Comparative Example 3.
A lithium secondary battery was manufactured in the same manner as in Comparative Example 2, except that VC and PS were not added during the preparation of the non-aqueous electrolyte solution.

Comparative Example 4.
A lithium secondary battery was manufactured _{in the same} manner as in Comparative Example ₁ , except that _LiCoO2 was used instead of _{LiNi0.85Co0.05Mn0.07Al0.03O2} as the positive electrode active material during the manufacture of the positive electrode.

Experimental Example 1. High-Temperature Life Evaluation Each of the lithium secondary batteries fabricated in the Examples and Comparative Examples was subjected to an activation process at 25° C. and a 0.2 C rate, and then gas was removed from the battery by a degassing process. The degassed lithium secondary batteries were subjected to constant current/constant voltage (CC/CV) charging at 0.33 C rate to 4.2 V and 0.05 C cut-off charging at 45° C., and constant current (CC) discharging at 0.33 C rate to 2.5 V.

One cycle of each of the above charge/discharge cycles was considered to be one cycle, and the discharge capacity in the initial state (one cycle) was measured and the results are shown in Table 1 below as the initial capacity. After repeating the same charge/discharge cycle 200 times, the discharge capacity was measured and the retention rate relative to the initial capacity was calculated and shown in Table 1 below as the capacity retention rate.

Experimental Example 2. High-Temperature Storage Evaluation Each of the lithium secondary batteries prepared in the Examples and Comparative Examples was subjected to an activation process at 25°C and a 0.2C rate, and then gas inside the battery was removed by a degassing process. Then, the batteries were charged at room temperature (25°C) at a 0.33C rate up to 4.2V under constant current/constant voltage conditions, followed by a 0.05C cut-off charge, and then discharged to 2.5V at 0.33C. The discharge capacity measured at this time is listed as the initial capacity in Table 1 below.

The lithium secondary battery was then stored at 60°C for four weeks, then transferred to a charger/discharger at room temperature (25°C) and charged at a 0.33C rate under constant current/constant voltage conditions up to 4.2V, followed by a 0.05C cut-off charge, and then discharged at 0.33C to 2.5V. The measured discharge capacity and retention rate relative to the initial capacity are shown in Table 1 below.

The results in Table 1 confirm that the batteries of Examples 1, 2, 5, and 6, which were manufactured using an electrolyte containing the compound represented by Chemical Formula 1 of the present application, were superior in both high-temperature life characteristics and high-temperature storage characteristics compared to the battery of Comparative Example 1, which was manufactured using an electrolyte not containing the compound represented by Chemical Formula 1, and the battery of Comparative Example 2, which was manufactured using an electrolyte containing coumarin instead of the compound represented by Chemical Formula 1.

Comparing the batteries of Example 3 and Comparative Example 3, both of which were manufactured using electrolytes containing neither VC nor PS, it can be seen that the battery of Example 3, which contains the compound represented by Chemical Formula 1, has superior high-temperature life characteristics and high-temperature storage characteristics.

Furthermore, even when the positive electrode material is changed to LCO, by comparing the results of Example 4 and Comparative Example 4, it can be confirmed that by including the compound represented by Chemical Formula 1 in the electrolyte, it is possible to improve the high-temperature life characteristics and high-temperature storage characteristics.

Claims (10)

A lithium salt,
an organic solvent;
A compound represented by the following chemical formula 1,
and one or more additives selected from vinylene carbonate, vinyl ethylene carbonate, 1,3-propane sultone, ethylene sulfate, lithium difluorooxalatoborate, and lithium difluorophosphate .
(In the above Chemical Formula 1,
R1 is an alkyl group having 1 to 10 carbon atoms,
R2 is an alkylene group having 1 to 10 carbon atoms,
L is a direct bond, —O—, —COO—, —RO—, or —R′COO—;
R and R' each independently represent an alkylene group having 1 to 10 carbon atoms,
m is an integer from 0 to 5. The compound represented by Chemical Formula 1 includes at least one compound selected from the group consisting of compounds represented by the following Chemical Formula 1-1 and compounds represented by the following Chemical Formula 2-1:
(In the chemical formula 1-1 and the chemical formula 2-1,
R1, R2, L, and m are as defined in Chemical Formula 1 above. The nonaqueous electrolyte solution for a lithium secondary battery according to claim 1, wherein L in Chemical Formula 1 is -O- or -COO-. The compound represented by Chemical Formula 1 includes at least one selected from the group consisting of a compound represented by the following Chemical Formula 1-2, a compound represented by the following Chemical Formula 1-3, a compound represented by the following Chemical Formula 2-2, and a compound represented by the following Chemical Formula 2-3.
(In the chemical formulas 1-2, 1-3, 2-2, and 2-3, R1, R2, and m are as defined in the chemical formula 1.) 2. The nonaqueous electrolyte solution for a lithium secondary battery according to claim 1, wherein the compound represented by Chemical Formula 1 comprises at least one selected from the group consisting of a compound represented by Chemical Formula 1A below, a compound represented by Chemical Formula 1B below, a compound represented by Chemical Formula 2A below, and a compound represented by Chemical Formula 2B below:
The non-aqueous electrolyte solution for a lithium secondary battery according to claim 1, wherein the content of the compound represented by Chemical Formula 1 is 0.1 wt % to 5 wt % based on the total weight of the non-aqueous electrolyte solution. The non-aqueous electrolyte solution for a lithium secondary battery according to claim 1, wherein the content of the compound represented by Chemical Formula 1 is 0.1 wt % to 1 wt % based on the total weight of the non-aqueous electrolyte solution. a positive electrode including a positive electrode active material;
a negative electrode including a negative electrode active material;
a separator interposed between the positive electrode and the negative electrode;
A lithium secondary battery comprising the nonaqueous electrolyte solution according to claim 1 . 9. The lithium secondary battery of claim 8 , wherein the positive electrode active material comprises a lithium composite transition metal oxide represented by the following Chemical Formula 3:
[Chemical formula 3]
Li _1+x ( _Nia Co _b Mn _c M _d ) O ₂
(In the above chemical formula 3,
M is one or more selected from W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo;
1+x, a, b, c, and d are the atomic fractions of Li, Ni, Co, Mn, and M, respectively;
0≦x≦0.2, 0.50≦a<1, 0<b≦0.25, 0<c≦0.25, 0≦d≦0.1, a+b+c+d=1.) 10. The lithium secondary battery according to claim 9 , wherein a, b, c, and d in Chemical Formula 3 are in the ranges 0.80≦a≦0.95, 0.025≦b≦0.15, 0.025≦c≦0.15, and 0≦d≦0.05, respectively.